Colorado mountain streams impacted by N deposition are net sources of N2O and CH4 to the atmosphere.

Mountain streams and lakes across the front range of Colorado are increasingly saturated with N in the form of NO3 from deposition. These historically oligotrophic ecosystems have responded to these changes in a number of ways that have been documented in previous work, but new effects of this deposition continue to appear This added N is likely responsible for new algal blooms that have been observed in lakes above the tree line in recent years. Streams and lakes in this region are often C limited, with excessive dissolved NO3 being exported downstream before being processed. What is uncertain, however, is how the microbial communities in these streams and lakes will respond to an increase of available C from decaying algae. In this study, we collected longitudinal stream emission data from a high elevation watershed impacted by N deposition where algal blooms were being observed for the first time. We additionally conducted incubations of stream sediment and water that were amended with live and dead algal cells to determine potential implications of a sudden influx of C. All stream reaches sampled were net sources of both CH4 and N2O to the atmosphere throughout the open water season, but the magnitude of these emissions appears to be tied to stream discharge. Incubated sediments demonstrated an increased potential for the production of CH4 and a weak increase in the potential production of N2O. These results suggest that N impacted streams have some degree of resiliency to changes in the availability of both C and N. With the continued decrease and stabilization of N deposition, these streams may revert to their pre-impacted status.